I. teaching material analysis
The concept of magnetic field is abstract, so it is very important for students to understand several common magnetic fields and learn this section well for the analysis of magnetic field force.
Second, the teaching objectives
Knowledge and skills
1. Know what a magnetic induction line is.
2. Know the distribution of several common magnetic fields (strip, hoof, linear current, annular current, energized solenoid) and magnetic induction lines.
3. Ampere's law will be used to judge the direction of linear current, annular current and magnetic field of energized solenoid.
4. Know the Ampere Molecular Current Hypothesis and explain related phenomena.
5. Understand the concept of uniform magnetic field, and make clear the uniform magnetic field in two cases.
6. Understand the concept of magnetic flux and be able to make relevant calculations.
(2) Process and method
Through experiments and students' hands-on (using ampere rule) and analogy, we can deepen our understanding of the basic knowledge of this section.
(3) Emotional attitudes and values
1. Further cultivate students' experimental observation and analysis ability.
2. Cultivate students' spatial imagination.
Third, the focus and difficulties in teaching
1. Ampere's law will be used to determine the direction of linear current, annular current and magnetic field of energized solenoid.
2. Understand the concept of magnetic flux correctly and make relevant calculations.
Fourthly, the analysis of learning situation.
The concept of magnetic field is abstract, which is difficult for students to understand, but electric field has been learned before, and students can be guided to learn by analogy.
Teaching methods of verbs (abbreviation of verb)
Experimental demonstration method and teaching method
Six, preparation before class:
Magnets and iron filings for demonstrating magnetic induction wires, and slides for demonstration.
Seven. Class schedule: 1 class hour
Eight, the teaching process:
(A) preview the inspection, summing up the doubts
(2) Introduction and display of the scene
Emphasis: the magnitude and direction of magnetic induction intensity B.
[Inspire students to think] The electric field can be vividly described by electric field lines. What can be used to describe the magnetic field?
[Student's answer] The magnetic field can be vividly described by magnetic induction lines. -introducing new courses.
(Teacher) Analog electric field lines can well describe the magnitude and direction of electric field intensity. Similarly, magnetic induction lines can also be used to describe the magnitude and direction of magnetic induction intensity.
(3) cooperative inquiry and intensive reading on demand
Blackboard 1. Magnetic induction line
Definition of (1) magnetic induction line
Draw some curves in the magnetic field so that the tangent direction of each point on the curve is consistent with the direction of magnetic induction intensity of that point. This curve is called magnetic induction line.
(2) Features:
A, the magnetic induction line is a closed curve, the magnetic induction line outside the magnet comes out from the north pole and returns to the south pole of the magnet, and the internal magnetic induction line goes from the south pole to the north pole.
B, each magnetic induction line is a closed curve, and any two magnetic induction lines do not intersect.
C the tangent direction of each point on the magnetic induction line indicates the magnetic field direction of that point.
D, magnetic induction linear density indicates the magnitude of magnetic induction intensity.
Demonstrate how to simulate the shape of magnetic induction line with iron filings and deepen the understanding of magnetic induction line. At the same time, it is compared with the electric field line.
Note (1) There is no magnetic induction line in the magnetic field, which is just imagined by people for the convenience of studying problems.
② The difference between electric field lines and magnetic induction lines is that electric field lines are not closed, while magnetic induction lines are closed curves.
2. Several common magnetic fields
certificate
① Demonstration experiment of simulating magnetic induction lines with iron filings, so that students can intuitively understand the distribution of magnetic induction lines (the direction and density distribution of magnetic induction lines) of bar magnets, hoof magnets, electrified straight wires, electrified annular currents, electrified solenoids and geomagnetic fields (simplified as a big bar magnet).
② Show them one by one with slides: bar magnet (figure 1), shoe magnet (figure 2), electrified straight wire (figure 3), electrified annular current (figure 4), electrified solenoid and geomagnetic field (simplified as a big bar magnet) (figure 5).
(1) Distribution of magnetic induction lines around the magnetic fields of bar magnets and hoof magnets with the same name and different names (Figure 1 and Figure 2)
(2) Current magnetic field and Ampere's law
① Magnetic field around linear current
On the basis of guiding students to analyze and summarize, draw a conclusion
Magnetic induction line around linear current: concentric circles centered on points on the conductor, all of which are on the plane perpendicular to the conductor (Figure 3).
B the relationship between the direction of linear current and the direction of magnetic induction line can be judged by ampere rule (also called right-hand spiral rule): hold the wire with the right hand, so that the direction pointed by the straight thumb is consistent with the direction of current, and the direction pointed by the bent four fingers is around the magnetic induction line.
② Magnetic field of annular current
The magnetic induction line of the annular current magnetic field is a closed curve around the annular conductor. On the central axis of the annular conductor, the magnetic induction line is perpendicular to the plane of the annular conductor (Figure 4).
[Teachers guide students]
B The relationship between the direction of annular current and the direction of magnetic induction line on the central axis can also be judged by Ampere's law: let the four fingers of the right hand bend in line with the direction of annular current, and the direction pointed by the straight thumb is the direction of magnetic induction line on the central axis of annular conductor.
③ Magnetic field of energized solenoid.
A magnetic induction line of energized solenoid magnetic field: similar to the magnetic induction line outside the bar magnet, one end is equivalent to the south pole and the other end is equivalent to the north pole; The inner magnetic induction line is parallel to the solenoid axis, pointing from the south pole to the north pole, and connected with the outer magnetic induction line, forming some closed curves around the current (Figure 5).
B The relationship between the current direction of the energized solenoid and the direction of its magnetic induction line can also be judged by Ampere's Law: Hold the solenoid with the right hand so that the direction pointed by the four fingers is consistent with the current direction, and then the direction pointed by the thumb is the north pole of the solenoid (the direction of the magnetic induction line inside the solenoid).
③ Characteristics of current magnetic field (compared with natural magnet): The presence or absence of magnetic field is controlled by switch; The direction of current can change the polarity of magnetic field; The strength of the magnetic field can be controlled by the current.
It shows that the following ampere force, Lorentz force and electromagnetic induction are closely related to the magnetic induction intensity, and the distribution of magnetic induction lines of several common magnetic fields is a very basic content, which will have a great influence on the later learning if it is not mastered well.
3. Ampere molecular current hypothesis
(1) Ampere Molecular Current Hypothesis
For molecular current, combined with the knowledge of magnetic field generated by annular current and ampere rule, it is easier for students to understand the sentence "its two sides are equivalent to two magnetic poles"; It should be emphasized that "these two magnetic poles are inextricably linked with molecular current", so as to understand why magnetic poles cannot exist as a single N pole or S pole.
(2) Some problems that can be explained by Ampere Hypothesis.
You can use paper clips, alcohol lamps, bar magnets and magnetizers to demonstrate magnetization and demagnetization, which will deepen students' impression. Give examples in life, such as magnetic cards can't be put together with magnets and so on.
Explaining "hypothesis" is a proposition that is used to explain a phenomenon but has not been confirmed by practice. In the process of establishing physical laws and theories, "hypothesis" often plays a very important role, which is summarized and abstracted on the basis of certain observations and experiments. The ampere molecular current hypothesis is inspired by Oster's experiment and produced through the development of thinking.
(3) Electrical property of magnetic phenomenon: The magnetic field of magnet and current is essentially generated by moving charge.
4. Uniform magnetic field
(1) Uniform magnetic field: If the magnitude and direction of magnetic induction intensity are the same everywhere in a certain area of magnetic field, the magnetic field in this area is called uniform magnetic field. The magnetic induction lines of uniform magnetic field are parallel straight lines with equal spacing.
(2) Uniform magnetic field in two cases: that is, the magnetic field between two very close different magnetic poles except the edge part; When two parallel coils (Helmholtz coils) separated by a certain distance are energized, the magnetic field P87 in the middle area is as shown in Figures 3.3-7 and 3.3-8.
5. Magnetic flux
(1) Definition: The product of the magnetic induction intensity b and the coil area s is called the magnetic flux passing through this surface (an important basic concept).
(2) Expression: φ=BS
Note ① Pay attention to the conditions of magnetic flux calculation, that is, b is the magnetic induction intensity of uniform magnetic field or can be regarded as uniform magnetic field, and s is the projected area of coil area on the plane perpendicular to the magnetic field direction.
(2) magnetic flux is scalar, but there are positive and negative points, which can be explained by special cases.
(3) Unit: Weber, abbreviated as Wei, symbol Wb 1Wb = 1T? The second part of money supply
(4) Another definition of magnetic induction intensity (magnetic flux density): b = φ/s.
The above formula indicates that the magnetic induction intensity is equal to the magnetic flux passing through the unit area, with Wb/m2 as the unit (another unit of magnetic induction intensity). So:1t =1WB/m2 =1n/a? m
(3) Summary: Make a brief summary of the knowledge points in this section.
(D) reflection and summary, classroom testing
1. As shown in the figure, the small magnetic needle placed in the middle of the energized solenoid points to the right at rest. Try to determine the positive and negative poles of the power supply.
Analysis: the direction of the N pole of the small magnetic needle is the direction of the magnetic field there, so the direction of the magnetic induction line in the solenoid is from A to B. According to Ampere's law, it can be judged that the current flows from the C terminal and flows in from the D terminal, so the C terminal is the positive pole of the power supply and the D terminal is the negative pole.
Note: Don't mistake the B end of the solenoid for attracting the N pole of the small magnetic needle, thus judging that the B end is equivalent to the south pole of the bar magnet. The key is to distinguish the distribution of magnetic induction lines inside and outside the solenoid.
2. As shown in the figure, when the coil is energized, the north pole of the small magnetic needle points to the reader. Students decide the direction of water flow.
A: The current direction is counterclockwise.
(5) Make a counseling plan and assign homework.
Nine, blackboard writing design
Magnetic induction line: artificially drawn, which can vividly describe the magnetic field.
Several common magnetic fields: Ampere rule: let the four fingers of the right hand bend in the same direction as the circular current, and the direction pointed by the straight thumb is the direction of the magnetic induction line on the axis of the circular conductor.
Uniform magnetic field: the electric field intensity is equal and the direction is the same everywhere in the magnetic field. Its magnetic induction lines are parallel straight lines at equal intervals.
Magnetic flux: the product of b and s, also called magnetic flux density in Weber.
X. Teaching reflection
The content of this section is closely related to the content of the first section of this chapter. You can review the knowledge of the first lesson before entering the new class and add corresponding exercises in the learning process. Pay attention to demonstrations, such as magnets and iron filings of magnetic induction lines, and slides for demonstrations, so that students have a sense of image.
Elective course 3- 1 chapter 3
3.3 Several Common Magnetic Fields
Pre-class preview study plan
First, preview the target.
1. Know what a magnetic induction line is.
2. Know the distribution of several common magnetic fields (strip, hoof, linear current, annular current, energized solenoid) and magnetic induction lines.
3. Ampere's law will be used to judge the direction of linear current, annular current and magnetic field of energized solenoid.
Second, preview the content
1, magnetic induction line
The so-called magnetic induction lines are some directional lines drawn in the magnetic field. On these lines, the magnetic field direction of each point is the tangent direction of that point. Basic characteristics of magnetic induction line: (1) The density of magnetic induction line indicates the magnetic field. (2) The magnetic induction line is a non-intersecting, non-tangent and non-interrupted closed curve; Outside the magnet, pointing from; Inside the magnet, pointing from. (3) The magnetic induction line is a fictitious physical model that vividly describes the magnetic field, and it does not really exist in the magnetic field. We can't think that there is a magnetic field where there is a magnetic induction line, and there is no magnetic field where there is no magnetic induction line.
2. Ampere's Law
When judging the relationship between the direction of linear current and the direction of magnetic induction line, Ampere's rule is expressed as follows: hold the wire so that the direction pointed by the straight thumb is consistent with the direction of current, and the direction pointed by the bent four fingers is the winding direction; When judging the relationship between the annular current and the current direction of the energized solenoid and the direction of the magnetic induction line, it should be uniformly expressed as: let the four bent fingers point in the same direction, and the direction pointed by the thumb is the direction of the annular current or the magnetic induction line of the energized solenoid (here, the annular current is regarded as a coil).
Third, ask questions.
Classroom inquiry learning plan
I. Learning objectives
1. Know the Ampere Molecular Current Hypothesis and explain related phenomena.
2. Understand the concept of uniform magnetic field, and make clear the uniform magnetic field in two cases.
3. Understand the concept of magnetic flux and be able to make relevant calculations.
Second, the learning process
1, Ampere Molecular Current Hypothesis
(1) Ampere Molecular Current Hypothesis: There is a kind of annular current in atoms, molecules and other material particles, which makes each material particle tiny, and its two sides are equivalent to two.
(2) the electrical nature of magnetic phenomena: the magnetic field of a magnet, like the magnetic field of current, is composed of.
(3) Magnetic materials can be divided into materials and materials according to the difficulty of demagnetization after magnetization.
2. Uniform magnetic field
A magnetic field with the same magnetic induction intensity and the same everywhere is called a uniform magnetic field. The magnetic induction line of uniform magnetic field is a straight line.
3. Magnetic flux
(1) Definition: In a uniform magnetic field with a magnetic induction intensity of B and an area of S, if there is a plane perpendicular to the direction of the magnetic field, the product of B and S is called the magnetic flux passing through the area.
(2) Definition:
(3) Unit: abbreviation and symbol. 1Wb= 1T？ The second part of money supply
(4) The magnetic flux is scalar.
(5) The magnetic flux density is magnetic induction intensity B= 1T= 1.
Classroom inquiry learning plan
Example 1. There is a rectangular coil, and the plane of the coil forms an angle with the direction of the magnetic field.
As shown in the figure. Let the magnetic induction intensity be b and the coil area be s, then it will pass through.
What is the magnetic flux of the coil?
As shown, two soft irons are placed on the shaft of the solenoid.
When the solenoid is energized, the two soft irons will (fill in "attractive force",
"repulsion" or "weakness"), terminal a will induce poles.
Example 3. Magnets lose their magnetism at high temperature or when they are hit. According to ampere's molecular current hypothesis, the reason is ()
A, molecular current disappears; B, the direction of molecular current becomes almost the same.
C, the orientation of molecular current becomes disordered D, and the intensity of molecular current weakens.
Third, reflection and summary.
Fourth, in-class testing.
Practice and improve after class
1, and the tangent direction of each point on the magnetic induction line indicates this point. Magnetic induction lines qualitatively represent the strength of the magnetic field.
2. The magnetic induction line is from the pole outside the magnet (solenoid) to the pole, and from the S pole to the inner pole. This is different from the electric field line. Magnetic induction line
3. If the intensity and direction of magnetic induction are in a certain area, the magnetic field in this area is called uniform magnetic field. Its magnetic induction line is a straight line.
4. For a charged straight wire, the right thumb represents the direction, and the directions of the four bent fingers represent the direction.
For the annular current and energized solenoid, the right thumb represents the direction, and the directions of the four bent fingers represent the direction.
Practice and improve after class
1, according to the idea of ampere hypothesis, it is considered that magnetic field is produced by moving charge. If this idea is also applicable to the geomagnetic field, but at present there is no charge found on the earth that moves directionally relative to the earth, then judging from this, the earth should ().
A, negatively charged B, positively charged C, uncharged D, uncertain
2, about the magnetic flux, the following statement is correct ()
In a uniform magnetic field, the magnetic flux passing through the surface is equal to the product of the magnetic induction intensity and the surface area.
B. In a uniform magnetic field, the area of coil A is larger than that of coil B, so the magnetic flux passing through coil A must be larger than that passing through coil B..
C, put a coil at M and N. If the magnetic flux passing through the coil at M is greater than that at N, the magnetic induction intensity at M must be greater than that at N..
D, the same coil is placed in a place with high magnetic induction intensity, and the magnetic flux passing through the coil is not necessarily large.
3. When a single-turn rectangular coil with an area of 5.0× 10-2m2 is placed in a uniform magnetic field with a magnetic induction intensity of 2.0× 10-2T, what is the magnetic flux passing through the coil when the plane of the coil is perpendicular to the direction of the magnetic field?
4. As shown in the figure, a ring is sleeved outside the bar magnet. When the ring moves down from the N pole to the S pole of the magnet (), how does the magnetic flux through the ring change?
First, gradually increase
B, gradually reduce
C, gradually increase first, and then gradually decrease.
D, gradually reduce first, and then gradually increase.
Reference answer:
Classroom inquiry learning plan:
Example 1, BSsin example 2, attracting s
Example 3, c
Training: 1, magnetic field direction density
2. Close N-pole, S-pole and N-pole 3. Parallel at equal intervals.
4. Current magnetic field current around the magnetic induction line
Practice and improve after class:
1、A 2、D 3、 10-3Wb 4、C